The practicality of administering a given drug percutaneously on a continuous basis depends upon the concentration of drug in the blood that is required to provide the desired therapy, how permeable the skin is to the drug, and the amount of skin surface area that is available for administration. The available skin surface area, while theoretically not being limited, is for patient acceptance reasons typically confined to more than about 5 cm.sup.2 and less than about 100 cm.sup.2. With available area fixed in this range, the matter narrows to whether sufficient drug will pass through that much area to provide the desired therapy. If it will, it is simple to effectively administer the drug percutaneously. If, however, the inherent permeability of the skin to the drug is so high or so low that too much or too little drug will pass through that area of skin, the rate of administration of the drug to the skin must be controlled or the permeability of the skin to the drug must be increased, as the case may be, to make percutaneous administration practical. The present invention involves a situation in which the permeability of the skin to the drug is increased.
There is a great deal of literature concerning compounds that enhance the percutaneous absorption of drugs. Typically a given amount of the enhancer is applied to the skin together with a given amount of the drug in a formulation that has no ability to control the rates at which the enhancer and drug are administered to the skin surface. In such instances more drug and more enchancer are present at the skin surface than the skin can absorb. Thus both drug and enhancer pass through the skin at maximum rates which are likely to be in excess of that needed to provide the desired therapeutic result.
U.S. Pat. No. 4,031,894 suggests pre- or coadministration of percutaneous absorption enhancers in connection with the controlled percutaneous administration of scopolamine. The enhancer is applied to eliminate the stratum corneum as a rate-affecting barrier to scopolamine absorption. To do this the enhancer must elevate the skin permeability to a level at which scopolamine is capable of moving through the skin faster than it is being applied to the skin surface by the system described in the patent. Thus the scopolamine permeates through the skin at the rate at which it is administered to the skin surface. In this type of application the scopolamine administration rate is said to be controlled by the system rather than by the skin. Correlatively, the enhancer administration rate (when coadministered with scopolamine) may be either system-controlled or skin-controlled, but in either case is of such magnitude as to make the scopolamine administration rate system-controlled.
U.S. Pat. No. 3,797,494 describes bandages for administering drugs percutaneously in which the drug may be mixed with a transport agent that enhances the penetration of the skin by the drug. The main components of these bandages are a backing layer, a drug reservoir layer, a microporous membrane layer, and a contact adhesive layer. The patent indicates that the rate of drug administration is controlled by the rate at which drug diffuses from the reservoir through the microporous membrane. These bandages operate, therefore, in the same manner as those described in U.S. Pat. No. 4,031,894. The drug administration rate is controlled by the bandage rather than by the rate at which drug is absorbed by the skin.
U.S. Pat. No. 3,053,255 discloses a multilayer unit for administering drugs percutaneously that is composed of the following layers beginning with the one closest to the skin: a fibrous carrier layer in which the drug is absorbed; an impervious separator layer; a reservoir layer in which a liquid transport agent that is a solvent for the drug is absorbed; and an impervious cover layer. One or more wicks run between the reservoir layer and the carrier layer and serve as a conduit for the flow of transport agent from the former to the latter. In operation the transport agent flows from the reservoir layer to the carrier layer via the wicks. The transport agent dissolves drug absorbed in the carrier layer as it passes therethrough and then, together with dissolved drug, is absorbed by the skin. The cross-sectional area of the wicks "must be such that at least so much of the vehicle can flow therethrough (including the active agent dissolved therein) as will be absorbed by the skin from the active agent carrier." This quote indicates that the amounts of drug and transport agent presented to the skin are equal to or greater than the amounts that the skin can absorb.